Carb Problems

[Anthony P.]

Re: Carb Problems

Please 'splain how the PCV valve was "bad"?

You didn't mention the "flat spot" in the carb, off-idle before. Was it there before you replaced the "bad" PCV valve?

If you replaced the valve with a different one that was intended, then this can very possibly affect your idle/off idle A/F ratio! The amount of spring tension and/or the orifice size and rate-of-change will effect fuel metering.

The original problem can be many things in addition to a vac leak. Is there excess play in the primary throttle shaft? Is your carb linkage free, without binding? Lubed? Accelerator pedal interference?

Same problem is fairly common. I also put a stiffer return spring on my Holley, after all else failed to cure the "problem". It is somewhat better now, but idle speed consistency still does not match my '85 with TPI.

Joe,
I took the valve off and put my thumb over it and the car idled much smoother so I replaced it with a new one I had in the garage both are 736C which is correct for my application.

The flat spot off idle was happening before I changed the PCV valve. I just purchased this carb from a re builder and put it on the car Wednesday and I did notice the flat spot off idle.

My last carb had the same fast idle while driving problem (but no flat spot off idle) so I thought I would ask you guys to see what would cause that, then attack the flat spot off idle.

Anthony

[Joe C.]

Re: Carb Problems

Joe,
I took the valve off and put my thumb over it and the car idled much smoother so I replaced it with a new one I had in the garage both are 736C which is correct for my application.

The flat spot off idle was happening before I changed the PCV valve. I just purchased this carb from a re builder and put it on the car Wednesday and I did notice the flat spot off idle.

My last carb had the same fast idle while driving problem (but no flat spot off idle) so I thought I would ask you guys to see what would cause that, then attack the flat spot off idle.

Anthony

You mean, you pulled the still connected valve out of the grommet in the valve cover, right? If you removed the valve from the hose, and then plugged the hose with your finger, then that's what is supposed to happen. Removing the valve from the hose with the engine idling is tantamount to a huge vacuum leak.
If that's what happened, then that indicates that the valve was dirty, and temporarily stuck too far open, and in a position where the pintle was allowing too much bypass air. Put the old valve back on and see if it does the same thing. If it does, then the original valve was working properly and indicates an overly lean idle mixture, which likely explains your off-idle flat spot. I'm not an expert on the emissions control systems on your car, but it is harder to track a vacuum leak in engines with emission controls.

[Anthony P.]

Re: Carb Problems

You mean, you pulled the still connected valve out of the grommet in the valve cover, right? If you removed the valve from the hose, and then plugged the hose with your finger, then that's what is supposed to happen. Removing the valve from the hose with the engine idling is tantamount to a huge vacuum leak.
If that's what happened, then that indicates that the valve was dirty, and temporarily stuck too far open, and in a position where the pintle was allowing too much bypass air. Put the old valve back on and see if it does the same thing. If it does, then the original valve was working properly and indicates an overly lean idle mixture, which likely explains your off-idle flat spot. I'm not an expert on the emissions control systems on your car, but it is harder to track a vacuum leak in engines with emission controls.

Joe,

I removed the valve from the grommet still connected to the carb and put my thumb over the valve which caused the car to idle smoother. Then I installed the new valve and repeated what I had done with the old valve and there was no difference. Needless to say but the car idles much smoother with the new PCV valve.

The flat spot was there with the old valve also. I am going to attack this problem tomorrow.

If the idle mixture is too lean how can you correct this??? I don't feel like shipping this carb back to this guy and waiting 2 weeks for him to fix it & ship it back to me.

Thanks
Anthony

[Joe C.]

Re: Carb Problems

Joe,

I removed the valve from the grommet still connected to the carb and put my thumb over the valve which caused the car to idle smoother. Then I installed the new valve and repeated what I had done with the old valve and there was no difference. Needless to say but the car idles much smoother with the new PCV valve.

The flat spot was there with the old valve also. I am going to attack this problem tomorrow.

If the idle mixture is too lean how can you correct this??? I don't feel like shipping this carb back to this guy and waiting 2 weeks for him to fix it & ship it back to me.

Thanks
Anthony

OK, confirmed that the old valve was hung too far open, thus creating a vacuum leak of sorts and lean condition.
I'm not a Rochester expert, but the typical fixes are to adjust the idle mixture screws, raise the float level, and decrease the size of the primary metering rods.
If Rochester is like Holley, and I believe it is, then turning the idle mixture screws CCW will richen idle mixture.
Check all hoses, gasket mating surfaces for vacuum leaks. If spraying ether along intake manifold ports/gaskets and carb mounting flange, on a warm, not hot engine, results in an increase in idle speed, then you have located a vacuum leak. Sometimes, if the leak is bad enough, "choking" a warmed up engine by partially covering the carb air horn with your hand will also result in an increase in idle speed.

[Anthony P.]

Re: Carb Problems

OK, confirmed that the old valve was hung too far open, thus creating a vacuum leak of sorts and lean condition.
I'm not a Rochester expert, but the typical fixes are to adjust the idle mixture screws, raise the float level, and decrease the size of the primary metering rods.
If Rochester is like Holley, and I believe it is, then turning the idle mixture screws CCW will richen idle mixture.
Check all hoses, gasket mating surfaces for vacuum leaks. If spraying ether along intake manifold ports/gaskets and carb mounting flange, on a warm, not hot engine, results in an increase in idle speed, then you have located a vacuum leak. Sometimes, if the leak is bad enough, "choking" a warmed up engine by partially covering the carb air horn with your hand will also result in an increase in idle speed.

Joe,

Thank you for the information. I will try all those thing tomorrow and hopefully cure the flat spot. I ll report back.

Anthony